The present invention relates to an optical transmission apparatus which realizes an optical communication system capable of long distance transmission of a good-quality analog signal without repeating.
A light-emitting diode (to be referred to as an LED for brevity hereinafter) is conventionally used as a light source for transmitting an analog signal using an optical fiber. However, if an analog signal is to be transmitted for a long distance without repeating, the power loss is too great and available power becomes too small due to a low coupling efficiency between the LED and the optical fiber, so that satisfactory transmission may not be performed.
On the other hand, if an LD is used as a light source, a significantly high power is obtained in comparison with an LED, and long distance signal transmission is possible due to the high power of the LD and its high coupling efficiency with an optical fiber. Furthermore, recent LDs have good linearity.
When an analog signal is directly subjected to intensity modulation using an LD as a light source and the modulated signal is transmitted through a multimode fiber, satisfactory signal transmission cannot be performed due to modal noise generated by high coherency of the light source. In order to reduce the coherency of a light source, the high-frequency superposition method is generally considered to be effective. A high-frequency signal to be superposed may be a sinusoidal wave or a pulsed wave and is added to the analog signal to drive the LD. In the case of a sinusoidal wave, if a high-frequency current superposed is below a threshold current of the LD, no output light is obtained from the LD. Then, the levels of the input signal and the light output signal are no longer proportional so as to cause distortion. In the case of a pulsed wave, distortion may not be caused if the superposed current wave has an ideal rectangular shape. However, the repetition frequency of the superposed pulses is generally within the range between 100 MHz and 500 MHz, and a band width of up to about the fifth harmonic is required to apply an ideal rectangular wave. This requires incorporation of a drive circuit and an LD having a wide band width of up to 500 MHz or higher. If the upper limit of the band width is lower than this, the same problem as in the case of a sinusoidal wave is encountered, thus similarly resulting in distortion. This gives rise to degradation in the differential gain (to be referred to as DG for brevity hereinafter), and the differential phase (to be referred to as DP for brevity hereinafter) characteristics.
Moreover, a frequency range required for the drive circuit of the LD must encompass the frequency of a high-frequency signal to be superposed in addition to relatively low frequencies as in the case of a video signal band width. This results in a complex and high-cost circuit configuration. Furthermore, a circuit for compensating for the distortion component as described above is required.